Humidity sensing element and method of making the same



Oct. 18, 1966 H. F. WOHRER 3,279,255

HUMIDITY SENSING ELEMENT AND METHOD OF MAKING THE SAME Filed July 7, 1964 6 IN TN ":1"? H I HENQY Fl WOHRER United States Patent 3,279,255 HUMIDITY SENfiING ELEMENT AND METHOD OF MAKING THE SAME Henry F. Wohrer, Milwaukee, Wis, assignor to Johnson Service Company, Milwaukee, Wis, a corporation of Wisconsin Filed July 7, 1964, Ser. No. 380,880 16 Claims. (Cl. 73-437) This invention relates to improved varying dimension humidity sensing elements for use in humidity control and/ or indications systems and to a method of fabricating the element.

Humidity sensitive elements are sensitive to the changes in the moisture content of air and respond in the form of dimensional changes to variations in humidity. Generally, it is preferable for the dimensional change occurring in the humidity sensing element to be directly proportional to the variations in the relative humidity. In addition, the humidity sensing element should have a fast response to moisture changes, should be reproducible in order to obtain uniformity in element-to-element performance, should have a minimum hysteresis and should not be affected deletreiously by extreme conditions of humidity and temperature.

In the past, hair, horn, w ood, and in some cases, paper or parchment, have been used as humidity sensing elements. All of these humidity sensing elements are natural occurring materials and have certain inherent disadvantages, such as fragility and are often damaged in shipment. Most important, however, these materials show variations in performance from element-to-element due primarily to variations in grain and capillary structure. For example, wood elements obtained from two trees of the same kind may not have the same control characteristics, and further, wood elements from the same tree may show inconsistencies due to variations in grain and capillary structure in various portions of the tree.

As a further disadvantage, the conventional elements produced from natural materials are difficult to produce. This is particularly true of the horn element, for it requires a very precise operation to cut the horn material into thin layers of uniform thickness. Thus, it is difficult to obtain uniform performance from element-to-element and uniformity can only be obtained through very careful calibration. As a further disadvantage, elements of this type do not retain their original calibration after long term exposure to extremes of humidity and in many cases require recalibration.

The present invention is directed to a varying dimension, humidity sensing element which overcomes the inherent disadvantages of the prior art elements. More specifically, the humidity sensing element of the invention includes a moisture insensitive base portion, and a thin layer or coating of a Water permeable substance is bonded to the base and the coating contains finely divided particles of a moisture sensitive material.

As the coating, within which the moisture sensitive particles are embedded, is permeable to Water vapor, the water vapor in the atmosphere can pass through the coating by capillary action to contact the moisture sensitive particles which are embedded or dispersed within the coating.

On an increase in moisture in the atmosphere, the moisture sensitive particles. tend to absorb moisture and expand. The expansion of the moisture sensitive particles causes a deflection of howling of the base portion, and this deflection can be utilized to initiate or change a signal to thereby register the relative humidity or actuate a humidity control device.

3,279,255 Patented Get. 18, 1966 Since the moisture sensitive particles, as well as the outer layer or coating, are formed of chemically pure materials and are applied as uniform coatings, substantially identical performance can be achieved which minimizes calibration from element-to-element.

The use of Water permeable coating and the moisture sensitive particles provide a humidity sensing element having rapid response to humidity conditions, and a characteristic not affected by extremes of temperature or humidity. The element has very little hysteresis and is substantially more stable than the humidity sensing elements used in the past.

Other objects and advantages will appear in the course of the following description.

The drawings illustrate the best mode presently contemplated of carrying out the invention.

In the drawings:

FIG. 1 is a fragmentary cross sectional view of the humidity sensing element of the invention;

FIG. 2 is a schematic representation showing the use of the humidity sensing element in a pneumatic type humidity controlled device; and

FIG. 3 is a view similar to FIG. 1 showing the modified form of the invention.

FIG. 1 illustrates a humidity sensing element 1 comprising a base or a core 2, and a thin layer 3 of a moisture permeable material is bonded to one surface of the base 2 and contains finely divided particles 4 of a moisture sensitive material.

The base 2 can be formed of organic or inorganic materials and should have a relatively low coeflicient of thermal expansion, less than 7 10- /F./inch. It has been found that metals, such as steel, stainless steel, Invar and the like, can be used as the base 2. The thickness of the base 2 is not critical and depends on the relative thickness of the layer 3 and the required response. For normal applications the base has a thickness in the range of .002 to .003 inch.

The layer 3 is formed of a material which is permeable or porous to water vapor. While the material of layer 3 will not absorb water and thus is not moisture-sensitive, meaning that it will not change in dimension with changes in the relative humidity, it should have excellent water vapor porosity, so that the water vapor of the atmosphere can pass through the material of layer 3 by cap-illary action and contact the moisture sensitive particles 4 which are embedded within the layer.

It has been found that cellulse nitrate and cellulose nitrate derivatives can be used as the layer 3. More specifically, pyroxylin, which consists chiefly of cellulose tetra nitrate, has proven very satisfactory as the layer 3. This material is soluble in solvents such as methanol, acetone, alcohol and ether, acetic acid, amyl acetate, methyl ethyl ketone and the like. Pyroxylin is normally used in the form of a solvent solution commonly referred to as collodion, which is generally considered to be a solution of 4 grams of pyroxylin in ml. of a mixture of 1 volume of alcohol and 3 volumes of ether. Collodion is a colorless, syrupy liquid and on evaporation of the solvent, leaves a tough water permeable film.

The thickness of layer 3 is not critical and for most applications, the layer 3 will have a thickness in the range of .020 to .035 inch. The thickness of the layer 3 does bear a relation to the thickness of base 2. For an element requiring rapid response, the thickness of the layer 3 should be between 8 to 14 times the thickness of the base with about 10 to 12 times being preferred. However, this relationship is variable and the optimum thickness ratio for a given base and layer 3 is dependent on 3 the particular application and is generally arrived at experimentally.

The moisture sensitive particles should be formed of a material which has a high sensitivity to moisture and will respond in the form of dimensional changes to moisture changes. Generally, the particles 4 should have a moisture sensitivity such that the particles will show a dimensional increase of at least 3%, and preferably 4% to 7%, with a change from to 100% relative humidity. These sensitivity values are based on the particles disassociated from the layer 3, and the change in dimension need only be in one dimension.

Generally speaking, the particles 4 are formed of a material characterized by having molecular chains of long bulky repeat units, inhibited rotation of the chain segment and polar groups, such as hydroxyl, carboxyl, imino and amino. These characteristics result in repeatable dimensional changes, high diffusional transport rates for water and dependence of linear dimension on relative humidity independent of temperature.

Specific examples of moisture sensitive materials which can be used as particles 4 are bone dust, animal horn, keratin, collagen, rice powder, starch and starch derivatives, regenerated protein, such as casein and zein, regenerated cellulose, and the like.

Depending on the desired sensitivity, the particles 4 of moisture sensitive material usually comprises from 50 to 80% by weight of the total weight of the layer 3 and particles. Elements in which the particles 4 comprise less than 50% by weight of the total weight can be fabricated but the sensitivity of such an element is low and thus not practical for use in many applications.

To improve the adhesion of the layer 3 to the base 2, the base can be provided with a series of holes 5. The holes 5 are preferably formed in a manner such that sharp bur-rs 6 extend outwardly from the surface of the base 2 and the layer 3 surrounds the burrs and partially fills the holes 5, and this substantially improves the adherence of the layer 3 to the base 2.

As the moisture-porous material 3 may expand and contract in accordance with temperature changes and thus tend to bow or deflect base 2, it is desirable to apply a coating 7 to the opposite surface of the base 2 which will compensate for any expansion and contraction of the layer 3 due to temperature changes. The coating 7 can be any organic or inorganic material which has a coeftficient of thermal expansion substantially similar to that of the layer 3. It has been found, when using cellulose nitrate, such as pyroxylin as the layer 3, that a coating 7 of pure pyroxylin or zinc will balance the expansion and contraction of the pyroxylin layer 3. The thickness of the coating 7 should generally be in the range of 0.0002 to .0010 inch with a coating of about .0005 inch being preferred. The total thickness of base 2 and coating 7 should generally not exceed 0.0030 inch. On changes in temperature, the layer 3 and coating 7 of zinc or other material will tend to bow the strip or base 2 in the opposite directions with the result that the coatings 3 and 7 act against the other so that no deflection occurs in the base due to temperature variations.

To prepare the humidity sensing element 1, the base strip 2 is initially cleaned in any desired manner to remove any foreign material or grime from the surface of the strip. The strip is then plated on both sides with the zinc or other coating 7. After plating, one zinc coated surface is masked and the exposed zinc coating is removed from the strip by an acid. Following this, the masking material is removed and the strip is punched to provide the holes 5. The strip is then ready to receive the water permeable layer 3.

The finely divided particles of the moisture sensitive material 4 are washed in a solvent, such as acetone, to remove foreign material and are then screened with the particles passing through a 100-mesh screen being suitable 4 for use in the product. The screened particles are then added to the solvent solution of the moisture permeable material and mixed therein to prevent settling.

The resulting suspension is applied to the uncoated surface of the base 2 by brushing, spraying or the like. It has been found that the coating can be brushed on in a series of coats. It is preferred to dry each coat before the subsequent coat is applied, and the drying can be conveniently accomplished by inserting the coated strip into an oven for several minutes to evaporate the solvent. Generally, from 10 to 20 coats of the suspension are separately applied to the surface of the base 2.

FIG. 2 is a schematic representation showing the use of the element 1 in a conventional, pneumatic-type humidity device. One end of the element 1 is clamped to a fixed support 8 and the opposite end is in engagement with or in close proximity to a nozzle 9 to restrict the air flow through the nozzle. Varying the air flow through nozzle 9 causes the air pressure within line 10 to vary, and the resulting pneumatic signal can be used to indicate relative humidity directly through a gauge 11 or to provide a mechanical input to humidification equipment through a pressure responsive member, such as bellows 12.

The porous nature of the layer 3 permits the water vapor in the atmosphere to penetrate by capillary action through the layer 3 so that the moisture can be absorbed in the particles 4 which are embedded within the layer 3. As the particles 4 expand, due to an increase in humidity, the expansion of the particles 4 will act to how the base 2 toward the nozzle 9 to restrict the air flow through the nozzle. On a decrease in humidity conditions, the particles 4 will tend to shrink and the shrinkage of the particles 4 will tend to bow the base 2 outwardly away from the nozzle, thereby increasing the airiiow through the nozzle and correspondingly varying the air flow in the line 10.

FIG. 3 represents a modified form of the invention in which th moisture sensitive element 13 includes an expandable base 14 which is formed of wire mesh or woven fabric, preferably cut on the bias so that the base can, expand and contract linearly. A layer of a water-porous material 15, similar to the layer 3, is applied to one surface of the mesh base 14, and the layer contains a plurality of moisture sensitive particles 16 similar to particles 4 of the first embodiment. It may be desired with the element 13 of FIG. 3, to attach a conventional temperature compensating mechanism to compensate for the minor expansion and contraction of the layer 15 due to temperature variations.

The humidity sensing element of FIG. 3 operates in a manner similar to that of the device of FIG. 1 except that the base 14 is moved linearly rather than tending to bow, as described with respect to FIG. 1. On an increase in moisture conditions, the particles 16 will tend to abs-orb moisture and expand, which thereby tends to expand or lengthen the expandable base member 14. Similarly, on a decrease in moisture condition, the particles 16 will contract, which provides a force on the base member to produce linear contraction of the base member. The linear expansion and contraction of the element 13 can be employed to initiate or change a signal to thereby register the relative humidity or actuate a humidity control device in a conventional manner.

The invention may be described in greater detail with reference to the following examples, which are meant to be illustrative but not limiting.

Example I A steel strip having a thickness of .002 inch was plated on both surfaces with zinc with the zinc coatings each having a thickness of .00025 inch. One zinc coated surface was marked with a conventional marking lacquer and the strip was exposed to hydrochloric acid to remove the exposed zinc coating. The strip was then immersed in lacquer thinner to remove the masking and 2 minutes to dry the coating.

subsequently punched to provide a series of inch holes spaced A inch apart.

A fihn casting solution was made by adding 2 grams of rice powder and 1 gram of bone powder to 3 grams of collodion diluted with 15 cc. of acetone. The mixture was stirred vigorously to completely disperse the rice and bone powder in the collodion solution.

The casting solution was applied by brushing to one surf-ace of the strip. After brushing, the coated steel was heated in an oven at a temperature of 110 F. for 19 subsequent coatings were applied to the steel surface and dried in the identical 7 manner to provide a final dried coating .025 inch thick.

Example 11 A casting solution was prepare-d by adding 2 grams of rice powder, gram of bone powder and 2 /2 grams of pyroxylin to a collodion solution consisting of 1 gram of collodion and 15 cc. of acetone. The mixture was stirred -to completely disperse the rice and bone powder in the solution.

T'wenty coats of the casting solution were separately applied to a zinc plated steel strip prepared in the manner described with respect to Example I provided a coating .025 inch thick.

The resulting element was mounted in an air stream of carefully controlled relative humidity and temperature, and on variations in relative humidity showed a minimum hysteresis and a rapid response.

Various modes of carrying out the invention are con templated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. A variable dimension humidity sensing element, comprising a first section being relatively insensitive to moisture and having a low coefiicient of thermal expansion, a second section bonded to the first section and composed of a Water vapor permeable material relatively insensitive to moisture, and a plurality of finely divided particles of a moisture sensitive material dispersed through said second section, said particles characterized by the ability to respond in the form of dimensional changes to changes in relative humidity.

2. A humidity sensing element, comprising a first section formed of a material having a low moisture sensitivity such that the material will show a dimensional increase of less than 2% of its initial dimension with a change in relative humidity from 0 to 100%, a second section bonded to the first section and having a low moisture sensitivity such that said second section will show a dimensional increase of less than 2% of its initial dimension with a change of relative humidity of 0 to 100% and said second section being characterized by the ability to permit water vapor to pass freely therethrough, and a plurality of finely divided particles of a moisture sensitive material embedded within and bonded to said second section, said particles formed of a substance having a high moisture sensitivity such that the substance will show a dimensional increase of more than 3% of its initial dimension with a change of relative humidity from 0 to 100%.

3. A humidity sensing element, comprising a base member formed of a material relatively insensitive to moisture, a layer applied to one surface of the strip and formed of a substance being relatively insensitive to moisture and capable of permitting water vapor to pass freely therethrough, a plurality of finely divided particles of a moisture sensitive material embedded within and bonded to said layer, said moisture sensitive material being capable of showing a dimensional increase of more than 3% of its initial dimension with a change of relative humidity from 0 to 100%, and a coating applied to the opposite surface of said base member, said coating having a coefiicient of thermal expansion substantially similar to the coeflicient of thermal expansion of said layer whereby expansion and contraction of said coating balances the expansion and contraction of said layer under thermal variations.

4. A humidity sensing element, comprising a metal strip having a thickness in the range of .002 to .003 inch, a layer of a substance having a high rate of water vapor permeability bonded to one surface of the strip, said substance being composed primarily of cellulose nitrate, a plurality of finely divided particles of a moisture sensitive material embedded within and bonded to said substance, said moisture sensitive material being capable of showing a dimensional increase of more than 3% of its initial dimension with a change of relative humidity from 0 to 100%, and means associated with the opposite surface of said base member for balancing for any changes in dimension of said substance due to variations in temperature.

5. A humidity sensing element, comprising a metal strip having a thickness in the range of .002 to .003 inch, a layer of a substance having a high rate of water vapor permeability bonded to one surface of the strip, said substance being composed primarily of cellulose nitrate, and a plurality of finely divided particles of a moisture sensitive material embedded within and bonded to said substance, said moisture sensitive material being capable of showing a dimensional increase of more than 3% of its initial dimension with a change of relative humidity from 6. The structure of claim 4, and including means for mechanically bonding said layer to the strip.

7. A humidity sensing element, comprising a base formed of a material relatively insensitive to moisture such that the material will show a dimensional increase of less than 2% of its original dimension with a change of relative humidity from 0 to 100%, a coating of a substance having a high rate of water vapor permeability bonded to a surface of the base, and a plurality of finely divided particles of a moisture sensitive material embedded within and bonded to said substance, said moisture sensitive material being capable of showing a dimensional increase of more than 3% of its initial dimension with a change of relative humidity from 0 to 100%, said material comprising from 50 to by weight of the total weight of said coating and said material.

8. The structure of claim 7 in which the base has a coefiicient of thermal expansion less than 7 X l0 F./ inch.

9. The structure of claim 2 in which the second section has a thickness from 8 to 14 times the thickness of the base member.

.10. A humidity sensing element, comprising a metal strip, a layer of pyroxylin bonded to one surface of the strip, and a plurality of finely divided particles of a mixture of bone powder and rice powder embedded within and bonded to said layer of pyroxylin, said pyroxylin having a high water vapor permeability permitting water vapor from the atmosphere to penetrate said layer and contact said particles, said particles being capable of expanding and contracting in dimension in accordance with changes in the relative humidity to thereby deflect said metal strip in accordance with changes in relative humidity.

1 1. The structure of claim 10, including a coating of zinc on the opposite surface of said metal strip with said zinc coating having a thickness less than .0010 inch.

12. A humidity sensing element, comprising a base member formed of a material relatively insensitive to moisture, a layer applied to one surface of the strip and formed of a material being relatively insensitive to moisture and capable of permitting water vapor to pass freely ear/9,255

therethrough, a plurality of finely divided particles of a moisture sensitive material embedded within and bonded to said layer, said moisture sensitive material formed of .a substance capable of showing a dimenisonal increase of more than 3% of its initial dimension with a change of relative humidity from to 100%, and a coating applied to the opposite surface of said base member, said coating having a coeflicient of thermal expansion substantially similar to the coefficient of thermal expansion of said layer whereby expansions and contractions of said coating balance the expansions and contractions of said layer under thermal variations, said base member being provided with a plurality of openings and said layer at least partially extending within said openings to firm-1y bond said layers to the base member.

13. A humidity sensing element, comprising a first section formed of a material having a low moisture sensitivity such that the material will show a dimensional increase of less than 2% of its initial dimension with a change in relative humidity from 0 to 100%, a second section bonded to the first section and having a low moisture sensitivity such that said second section will show a dimensional increase of less than 2% of its initial dimension with a change of relative humidity of 0 to 100%, and said second section being characterized by the ability to permit water vapor to pass freely therethrough, and a plurality of finely divided particles of a moisture sensitive material embedded within and bonded to said second section, said particles formed of a substance having a high moisture sensitivity such that the substance will show a dimensional increase of more than 3% of its initial dimension With a change of relative humidity from 0 to 100%, said first section having a series of surface deviations extending outwardly from a surface of said first section and said deviations being embedded within said second section to thereby firmly bond said second section to said first section.

14. The structure of claim 13, in which the deviations consist of a plurality of openings in said first section which terminate in burrs embedded within said second section.

'15. The method of making a variable dimension humidity sensing element, comprising the steps of mixing finely divided particles of a moisture sensitive material in a solvent solution of a water vapor permeable substance to form a mixture, applying the mixture to a surface of a base member, evaporating the solvent from said mixture to provide a coating of said water vapor permeable substance with particles of said moisture sensitive material embedded therein, and applying a coating of a mate-rial to the opposite surface of the base with said last named material having a coefficient of thermal expansion substantially similar to that of said Water permeable substance to thereby balance the expansion and contraction of said substance under thermal variations.

"16. A method of making a humidity sensing element, comprising the steps of mixing finely divided particles of a moisture sensitive material with a solvent solution of a water permeable substance to form a suspension, said moisture sensitive material being capable of showing a dimensional increase of more than 3% of its initial dimension with a change of relative humidity fr0m0 to 100%, applying the suspension to a surface of a metal strip, evaporating the solvent to provide a dry coating of said water permeable substance containing finely divided particles of said moisture sensitive material with said moisture sensitive material comprising from toby weight of the combined weight of said substance and said particles, and applying a coating of a material havinga coeflicien-t of thermal expansion substantially similar to said water permeable substance to the opposite surface of said metal strip.

References Cited by the Examiner UNITED STATES PATENTS 2,469,537 5/1949 Wohrer 73337 3,104,179 9/1963 Prior l17122 DAVID SCHONB-ERG, Primary Examiner.

LOUIS R. PRINCE, Examiner.

M. B. HEPPS, Assistant Examiner. 

1. A VARIABLE DIMENSION HUMIDITY SENSING ELEMENT, COMPRISING A FIRST SECTION BEING RELATIVELY INSENSITIVE TO MOISTURE AND HAVING A LOW COEFFICIENT OF THERMAL EXPANSION, A SECOND SECTION BONDED TO THE FIRST SECTION AND COMPOSED OF A WATER VAPOR PERMEABLE MATERIAL RELATIVELY INSENSITIVE TO MOISTURE, AND A PLURALITY OF FINELY DIVIDED PARTICLES OF A MOISTURE SENSITIVE MATERIAL DISPERSED THROUGH SAID SECOND SECTION, SAID PARTICLES CHARACTERIZED BY THE ABILITY TO RESPOND IN THE FORM OF DIMENSIONAL CHANGES TO CHANGES IN RELATIVE HUMIDITY. 